STRUCTURED LIPIDS: a Product with High Expectation

Structured Lipids: A Product with High Expectation

STRUCTURED LIPIDS: A PRODUCT WITH HIGH EXPECTATION

Xuebing Xu

BioCentrum-DTU, Technical University of Denmark, DK-2800 Lyngby, Denmark

Correspondence: Xuebing Xu, BioCentrum-DTU, Technical University of Denmark

Building 221, DK-2800 Lyngby, Denmark

Malaysian Oil Science and Technology 2005 Vol. 14 No. 21

Structured Lipids: A Product with High Expectation

Introduction

Structured lipids (SLs) generally refer to any fats that are modified or restructured from natural oils and fats or fatty acids therefrom, primarily containing short or medium chain fatty acids and preferably essential fatty acids. The oils are targeted to have special functionality or nutritional properties for edible or pharmaceutical purposes. This definition in fact covers all oils and fats containing short/medium chain fatty acids, whether produced by either chemical or enzymatic methods. The chain length of fatty acids is normally defined as long chain (more than 12), medium chain (8-12), and short chain (less than 8).

Structured lipids have been a hot topic for more than 10 years.1-3 The applications of SLs were initiated from the applications of medium chain triacylglycerols (MCT) in clinical treatment of fat malabsorption. Due to the nutritional requirements of essential fatty acids, natural vegetable oils containing essential fatty acids are often blended with MCT for medical applications. However, the physical mixtures of MCT and long chain triacylglycerols (LCT) are metabolized differently and result in the retention of the original absorption rates of the MCT and LCT. This led to the development of interesterified products between MCT and LCT.

Structured lipids: aspects of nutritional studies

The degradation process of oils and fats in human body is regiospecific and ideally results in the formation of sn–2 monoacylglycerols (MAGs) and free fatty acids. Free fatty acids liberated from dietary lipids during digestion are metabolized more rapidly if they are medium or short chain, whereas long chain fatty acids can be absorbed directly in the form of MAGs. This implies that the fatty acids located at the sn-2 position may have different metabolic paths compared to those at the 1,3-positions. This is important when considering the possible advantages of tailor-made fats with particular triacylglycerol structures. This issue has been in an intensive discussion and the interest has largely accelerated the development of enzyme technology for the production of regio-specifically defined products.

The interest in short or medium chain fatty acids is obvious. There are more easily released from oils and fats. The activity of pancreatic lipase has been extensively examined using in vitro conditions. The lipase will be active towards fatty acids located in the sn-1,3 positions. A number of SLs have been synthesised and tested as substrates by in vitro hydrolysis with pancreatic lipase. Jandacek et al.4 demonstrated that the SLs, 8:0/18:2/8:0 and 8:0/18:1/8:0, were hydrolysed as rapidly as MCT and more rapidly than oils with long chain fatty acids in all positions of the triacylglycerols (TAGs). For randomly interesterified fats containing 8:0 and 10:0 as well as 18:2n-6, hydrolysis decreased with increasing contents of 18:2n-6, indicating the higher lipase activity towards medium chain fatty acids. Minor amounts of plasma medium-chain fatty acids and no improved time trial performance after consuming specifically defined SLs were observed in a recent study.5

The absorption of SLs has been studied using well-defined oils or interesterified fats. Ikeda et al.6 examined the thoracic lymph absorption of SLs. They concluded that SLs would be better than MCT or LCT in the treatment of malabsorption. Tso et al.7 made the similar conclusion with a rat model of fat malabsorption. Jensen et al.8 compared a specifically structured oil with 18:2n-6 at the sn-2 position and 8:0 and 10:0 at the sn-1,3 positions and the same oil in a randomised form. They found that the lymph TAGs had the highest levels of 18:2n-6 after intake of the specific SL compared to the randomised SL. Jensen et al.9 compared a randomly interesterified fat manufactured from MCT and fish oil versus the equivalent mixture of the two fats for lymph absorption in a canine model. They found higher transport of medium chain fatty acids from the randomised fat compared with the physical mixture. Christensen et al.10 examined the specific SL and the randomised ones and found more rapid absorption of 20:5n-3 and 22:6n-3 from the former and more rapid absorption of 10:0 from the latter. This confirmed the importance of the conservation of the sn-2 position for n–3 polyunsaturated fatty acids (PUFA). Effect of SLs containing medium-chain fatty acids and linoleic acid on clearance rate in serum of triaclglycerols in rats was studied.11 The authors demonstrated that the structural differences in triacylglycerols containing medium-chain fatty acids and linoleic acid could alter the rate of lipid clearance in serum of rats.

The intestinal absorption of specifically defined SLs was examined and indicated that the medium-chain fatty acids from SLs, in addition to absorption into the portal blood as free fatty acids, are absorbed by the same pathway as the conventional long-chain triacylglycerols, that is, they are hydrolyzed into free fatty acids, absorbed and activated into CoA, and reacylated into triacylglycerols in the enterocyte.12 The chain length of medium chain fatty acids in the primary positions of SLs seems to have no effect on the maximal intestinal absorption of long-chain fatty acids in the sn-2 position in the rat model, whereas the distribution of fatty acids between the lymphatics and the portal vein reflects the chain length of the fatty acid.13

SLs have potential applications in the delivery of energy and PUFA to persons suffering from malabsorption. This has been convincingly demonstrated in animal models. Jandacek et al.4 applied an irrigated intestinal loop model to examine the absorption of fats. They found that 8:0/18:2n-6/8:0 was better absorbed than 18:1/18:2n-6/18:1. Christensen et al.14 demonstrated higher lymphatic transport of 18:2n-6 from a specifically structured fat compared to a randomised fat or the blended fat. A recent study shows that structured lipids improved fat absorption in normal and malabsorbing rats.15 The study demonstrated improved hydrolysis and absorption of the specific oil compared with the other oils examined both in rats with normal absorption and in rats with malabsorption. Lipid profiles in rats were found to be significantly affected after intake of highly purified SLs containing medium chain fatty acids and linoleic acid with specific locations.16 The results indicated that the feeding of highly purified LML types could effectively improve serum and liver lipid profiles and that MLM types might be a preferable substrate for the pancreas and contribute to energy supply in rats (M = medium chain fatty acids and L = long chain fatty acids).

Following the absorption of the sn-2 MAG, a resynthesis of TAGs for chylomicron production takes place in the intestinal cells. After intake of a normal TAG from fats containing long chain fatty acids, the pool of fatty acids absorbed from the intestine will be used for the resynthesis of TAGs. This process is stereospecifically-favouring acylation at the sn-1 position.17 For SLs, medium chain fatty acids will be released but more will be transferred to the portal vein and thus not be available for the resynthesis of TAGs. The fatty acids for resynthesis of TAGs must then be derived from the endogenous pools, i.e. either from the bile phospholipids or from the fatty acids transported to the intestine from liver or adipose tissue with VLDL, and the availability of fatty acids may in fact limit the absorption and intestinal resynthesis of TAGs,18 indicating a low fat accumulation in the body. Effect of SLs on serum triacylglycerol levels and body fat in college athletes was studied.19 The study shows that SLs, compared with soybean oil, may have the potential to prevent hypertriglyceridemia and obesity caused by consumption of a high-fat diet.

Uptakes of both PUFA and medium chain fatty acids will be favourable in several cases. In animal models of burn patients it has been demonstrated that healing and tissue regeneration was favoured by intake of the SL derived from MCT and fish oil.20 This can be attributed both to the increased demand for PUFA for tissue regeneration and to the intake of medium chain fatty acids, which, during absorption, will be directed towards the liver for oxidation and thereby spare the protein from utilisation for energy. Protein-sparing effects on protein and energy metabolism in the hypercatabolic state was also studied in a low dose endotoxin rat model with chemically defined SLs containing omega 3 or 6 fatty acids at the sn-2 position.21 Considerably different performance has been seen for SLs with different structures.

In parenteral nutrition the fats are given as an emulsion and the purpose is to provide PUFA and fat-soluble vitamins. The metabolism of SLs given as emulsions has been examined.22 Higher fractional clearance rate for the SLs was found, indicating that this was removed more rapidly from the circulation than the other emulsions. This agreed with similar findings for drug delivery systems based on SLs.23

The applications of emulsions will include total parenteral nutrition treatment of extensive burn wounds, where largest gains in body weight, greatest positive nitrogen balance and highest energy consumption was reported following the intake of an emulsion from SLs.24 In postoperative patients randomised SLs were rapidly metabolised compared with conventional emulsions for total parenteral nutrition.25 Also in infusion treatment of animals with implanted tumours, it has been observed that tumour growth was slowed down and muscle tissue restored in rats by feeding randomised SLs.26 SLs based on fish oil may have potential applications in the situation of rejection of transplants, endotoxic shock, and chronic and progressive inflammation by cancer.27

The use of SLs in clinical nutrition for now can be primarily concluded as providing energy as well as PUFA. We may add the purpose of directing the distribution of the components of the SLs towards different tissues in the body following absorption. In cases of normal absorption the purpose of applying SLs may be (a) to increase the rate of uptake of PUFA for tissue regeneration; (b) to supply rapidly absorbed energy from medium chain fatty acids; (c) to reduce the energy density of the fat through the lower energy content of medium chain fatty acids; (d) to direct the fatty acids towards the hepatic tissue for oxidation and to minimise deposition in the adipose tissue; (e) to provide fatty acids for immunosuppression using n-3 fatty acids from fish oils.

Collective evidences also show that only little difference has been observed for SLs, in particular with different structures, in terms of effects on serum and liver lipid profiles in rats,28 on safety and tolerance parameters as well as fatty acid distribution in phospholipids of hepatic and extrahepatic tissues,29 on memory and learning ability,30 and on chylomicron metabolism.31Differences in the intramolecular structure of SLs in a study did not affect pancreatic lipase activity in vitro or the absorption by rats of (n-3) fatty acids.32One very recent study also shows that attenuated gastric distress but no benefit to performance was observed with adaptation to the SLs from interesterification of rapeseed oil and tricaprylin in well-trained male and female cyclists (Thorburn et al. 2005, submitted). Another study with 3 fatty acids and caprylic acid but in different structures did not shown marginal difference on plasma concentrations of TAG and cholesterol, when fed as part of low-fat diets to rats (Porsgaard et al. 2005, submitted).

Structured lipids: availability

Availability of SLs tightly relates to the new discoveries and collection of evidence in nutritional and functional studies. Production of SLs can be done by either chemical or enzymatic interesterification or synthesis depending on what products are needed. Randomised SLs can be produced by both methods. However, SLs with defined structures can only be produced by the enzymatic method with specific lipases, especially in large quantities.

For most SL products for nutritional applications, PUFA and medium chain fatty acids are most important fatty acids to be considered. To combine these two types of fatty acids into SLs, natural or other available sources of different fatty acids are needed for the production. In some productions, the specific location of a fatty acid is necessary to obtain specific products and properties.

For randomized SLs, the easy way will be by implementing chemical interesterification between two oils with required fatty acid compositions. However, lipase-catalysed reactions between two oils are also on the way with balanced cost. The advantages of enzyme approaches are indeed tremendous, including (i) efficacy of lipases under mild reaction conditions; (ii) utility in “natural” reaction systems and products; (iii) reduced environmental pollution; (iv) availability of lipases from a wide range of sources; and (v) ability to improve lipases by genetic engineering. With the increasing attention of environmental protection and the customer demands of ‘green’ products, the interesterification between two oils with lipases is moving into industrial scale, especially for the products with nutritional considerations.

Research focus on production technology has been largely on SLs with defined structures or the application of enzyme technology, since chemical methods are mature in reality. Many excellent overall reviews on the enzymatic production or synthesis have been published.33-36 Technology has been advanced to the stage of pilot or even industrial operation in large quantities. However, commercial SLs, especially with defined structures, are not readily available, largely because the benefits and applications of SLs are not convincing enough to lead to a successful application.

On the other hand, functions of SLs with respect to the consideration of medium chain fatty acids or PUFA have gained wide recognition. A number of products, largely with randomized structure, are available for different applications. A collection and evaluation of such products is given in Table 1. It is for sure that new concepts of applications of SLs in foods or pharmacy are in high needs.

Malaysian Oil Science and Technology 2005 Vol. 14 No. 21

Structured Lipids: A Product with High Expectation

Table 1. Commercial structured lipids and their applications

Malaysian Oil Science and Technology 2005 Vol. 14 No. 21

Structured Lipids: A Product with High Expectation

Remarks

Structured lipids as a concept development has attracted intense interest for the last few years. Academic research has demonstrated a number of potential aspects from nutritional or biochemical points of view. With the potential perspectives in mind, technology development has already made the possibility of SL synthesis into production level with reasonably acceptable product standards. So far no industrial efforts have been claimed for the commercial production of SLs with defined structures, even though there have been considerable evidences showing the positive effects in a few aspects of functional applications. It is likely that nutritional considerations of many issues are far from convincing and remain an intensive topic in the research cycle. A general consensus for SLs remains for the effects of fatty acids in SLs rather than the effects of structure of SLs. This issue will take more years to be clarified. Intensive research programs are still moving on worldwide. A full picture of SLs in human nutrition and food functionality will emerge eventually sooner or later.

Acknowledgement

Danish Technological Research Council (STVF) and Center for Advanced Food Studies (LMC) are acknowledged for the supports.

References

1. Haumann BF (1997). Structured lipids allow fat tailoring, INFORM 8: 1004–1011.
2. Akoh CC (1998). Structured lipids. C.C. Akoh, D.B. Min, eds. In: Food Lipids: Chemistry, Nutrition, and Biotechnology. New York: Marcel Dekker, pp 699–727.
3. Høy C-E and X Xu (2001). Structured triacylglycerols, in: Structured and Modified Lipids, ed. F.D. Gunstone, Marcel Dekker, New York, pp. 209–240.
4. JandacekRJ, WhitesideJA, HolcombeBN, VolpenheinRA and TaulbeeJD (1987). The Rapid Hydrolysis and Efficient Absorption of Triglycerides with Octanoic-Acid in the 1-Position and 3-Position and Long-Chain Fatty-Acid in the 2-Position, American Journal of Clinical Nutrition 45: 940–945.
5. VistisenB, NyboL, XuXB, HoyCE and KiensB (2003). Minor amounts of plasma medium-chain fatty acids and no improved time trial performance after consuming lipids, Journal of Applied Physiology 95: 2434–2443.
6. IkedaI, TomariY, SuganoM, WatanabeS and NagataJ (1991).